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CSE401n:Computer Networks
Lecture 16
Wireless Link & LANs
WS: ch-14
KR: 5.7
1
IEEE 802.11 Wireless LANs
 802.11b
 operate at 2.4 GHz, 11 Mbps
 widely deployed
 802.11a
 5-6 GHz range
 up to 54 Mbps
 802.11g
 2.4 GHz
 up to 54 Mbps
 All have base-station and ad-hoc network
versions
 All use CSMA/CA for multiple access
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IEEE 802.11 Wireless LAN
 IEEE 802.11 standard:

unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz
and 802.11b
802.11a
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Frequency Band
4
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IEEE 802.11 Wireless LAN
 Basic Service Set (BSS)
(a.k.a. “cell”) contains:
 wireless station (WS)
 access point (AP): base
station
 BSS’s combined to form
distribution system (DS)
 Two operation modes:

Infrastructure mode
• everything through AP

Peer-to-peer mode
• called ad hoc network
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IEEE 802.11 Architecture
 Distribution system (DS)
 Access point (AP)
 Basic service set (BSS)
 Stations competing for access to shared wireless medium
 Isolated or connected to backbone DS through AP
 Extended service set (ESS)
 Two or more basic service sets interconnected by DS
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Ad Hoc Network approach
 No AP
 No connection to the outside world
 Wireless hosts communicate with each
other
 Applications:
 “laptop” meeting in conference room, car
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Distribution of Messages Within a
DS
 Distribution service
 Used to exchange MAC frames from station in one BSS to
station in another BSS
 Integration service
 Transfer of data between station on IEEE 802.11 LAN and
station on integrated IEEE 802.x LAN
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Transition Types Based On Mobility
 No transition
 Stationary or moves only within BSS
 BSS transition
 Station moving from one BSS to another BSS in same ESS
 ESS transition
 Station moving from BSS in one ESS to BSS within
another ESS
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Association-Related Services
 Association
 Establishes initial association between station and AP
 Reassociation
 Enables transfer of association from one AP to another, allowing
station to move from one BSS to another
 Disassociation
 Association termination notice from station or AP
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Access and Privacy Services
 Authentication
 Establishes identity of stations to each other
 Deathentication
 Invoked when existing authentication is terminated
 Privacy
 Prevents message contents from being read by unintended
recipient
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802.11 – Traffic and Access
Methods
 Two types of traffic services

Asynchronous Data Service (mandatory)
• exchange of data packets based on “best-effort”

Time-Bounded Service (optional)
 Two types of coordination function


DCF (Distributed Coordination Function)
PCF (Point Coordination Function)
 Access methods (why not CSMA/CD?)

DCF-CSMA/CA (collision avoidance)
• collision avoidance via randomized back-off mechanism (if sense
busy, random backoff even if no collision)
• ACK packet for acknowledgements (not for broadcasts)

DCF-RTS/CTS
• avoids hidden terminal problem

PCF
• access point polls
7.12.1
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IEEE 802.11 Medium Access
Control
 MAC layer covers three functional areas:
 Reliable data delivery
 Access control
 Security
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Reliable Data Delivery
 More efficient to deal with errors at the MAC level than
higher layer (such as TCP)
 Frame exchange protocol



Source station transmits data
Destination responds with acknowledgment (ACK)
If source doesn’t receive ACK, it retransmits frame
 Four frame exchange
 Source issues request to send (RTS)
 Destination responds with clear to send (CTS)
 Source transmits data
 Destination responds with ACK
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The Hidden Terminal Problem
A
B
C
 A is sending to B, but C cannot receive from A

Friis Law (power decay proportional to distance square)
 Therefore C sends to B, without detecting the transmission
from A to B
 In summary, A is “hidden” for C
 Implication: How to do carrier sense and collision detection?
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The Exposed Terminal Problem
A
B
C
D
 B is sending to A, C intends to send to D
 C senses an “in-use” medium, thus C waits
 But A is outside the radio range of C,
therefore waiting is not necessary
 In summary, C is “exposed” to B
 Implication: false carrier sense
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Basic Solution: RTS-CTS
 Short signaling packets
 RTS (request to send) and CTS (clear to send)
 Contain sender address, receiver address,
transmission duration
 Example: solve the hidden terminal
problem
A
RTS
CTS
B
CTS
C
D
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Access Control
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Medium Access Control Logic
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Interframe Space (IFS) Values
 Short IFS (SIFS)
 Shortest IFS
 Used for immediate response actions
 Point coordination function IFS (PIFS)
 Midlength IFS
 Used by centralized controller in PCF scheme when using polls
 Distributed coordination function IFS (DIFS)
 Longest IFS
 Used as minimum delay of asynchronous frames contending for access
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IFS Usage
 SIFS
 Acknowledgment (ACK)
 Clear to send (CTS)
 Poll response
 PIFS
 Used by centralized controller in issuing polls
 Takes precedence over normal contention traffic
 DIFS
 Used for all ordinary asynchronous traffic
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802.11 – Inter Frame Spacing
 Defined different inter frame spacing
 SIFS (Short Inter Frame Spacing)

highest priority, for ACK, CTS, polling response
 PIFS (PCF IFS)

medium priority, for time-bounded service using PCF
 DIFS (DCF, Distributed Coordination Function IFS)

lowest priority, for asynchronous data service
DIFS
DIFS
medium busy
PIFS
SIFS
contention
next frame
t
direct access if
medium is free  DIFS
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802.11 – RTS/CTS + ACK
 Sender sends RTS with NAV (Network allocation Vector, i.e.
reservation parameter that determines amount of time the data
packet needs the medium) after waiting for DIFS
 Receiver acknowledges via CTS after SIFS (if ready to receive)

CTS reserves channel for sender, notifying possibly hidden stations
 Sender can now send data at once, acknowledgement via ACK
 Other stations store NAV distributed via RTS and CTS
DIFS
sender
data
RTS
SIFS
receiver
other
stations
CTS SIFS
SIFS
NAV (RTS)
NAV (CTS)
defer access
ACK
DIFS
new contention
data
t
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802.11 Collision Avoidance: RTS-CTS + ACK
DIFS: Distributed Inter-Frame Spacing
SIFS: Short Inter-Frame Spacing
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802.11: PCF for Polling
PIFS
point
coordinator
D
D
SIFS
U
polled
wireless
stations
NAV
SIFS
NAV
medium
busy
contention free period
contention
period
t
D: downstream poll, or data from point coordinator
U: data from polled wireless station
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MAC Frame Format
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MAC Frame Fields
 Frame Control – frame type, control information
 Duration/connection ID – channel allocation time
 Addresses – context dependant, types include source and
destination
 Sequence control – numbering and reassembly
 Frame body – MSDU or fragment of MSDU
 Frame check sequence – 32-bit CRC
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Frame Control Fields
 Protocol version – 802.11 version
 Type – control, management, or data
 Subtype – identifies function of frame
 To DS – 1 if destined for DS
 From DS – 1 if leaving DS
 More fragments – 1 if fragments follow
 Retry – 1 if retransmission of previous frame
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Frame Control Fields
 Power management – 1 if transmitting station is in sleep mode
 More data – Indicates that station has more data to send
 WEP – 1 if wired equivalent protocol is implemented
 Order – 1 if any data frame is sent using the Strictly Ordered
service
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Control Frame Subtypes
 Power save – poll (PS-Poll)
 Request to send (RTS)
 Clear to send (CTS)
 Acknowledgment
 Contention-free (CF)-end
 CF-end + CF-ack
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Data Frame Subtypes
 Data-carrying frames
 Data
 Data + CF-Ack
 Data + CF-Poll
 Data + CF-Ack + CF-Poll
 Other subtypes (don’t carry user data)
 Null Function
 CF-Ack
 CF-Poll
 CF-Ack + CF-Poll
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Management Frame Subtypes
 Association request
 Association response
 Reassociation request
 Reassociation response
 Probe request
 Probe response
 Beacon
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Management Frame Subtypes
 Announcement traffic indication message
 Dissociation
 Authentication
 Deauthentication
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Wired Equivalent Privacy
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Authentication
 Open system authentication
 Exchange of identities, no security benefits
 Shared Key authentication
 Shared Key assures authentication
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IEEE 802.11
802.11a
802.11b
802.11
Standard
approved
Sep. 1999
Sep. 1999
July 1997
Available
bandwidth
300 MHZ
83.5 MHZ
83.5 MHZ
Unlicensed freq. 5.15-5.35G
of operation
5.725-5.825G
2.4-2.4835G
2.4-2.4835G
No. of nonoverlapping Ch.
4
3
3
Rate per
channel
6,9,12,18,24,36,
48,54 Mbps
1, 2, 5.5, 11
Mbps
1,2 Mbps
Range
225 feet
225 feet
??
Modulation
OFDM
DSSS
DSSS, FHSS
DSSS: direct sequence spread spectrum
FHSS: frequency hopping spread spectrum
OFDM: orthogonal frequency division multiplexing
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